1. Technical Field
This disclosure relates to a substrate structure, a method of fabricating the same, and light-emitting diode devices fabricated from the same.
2. Related Art
A type of homo-substrate includes an air-bridge structure where an epitaxial film is formed from and supported by crystal piers of the same nitride semiconductor material disposed on a hetero-substrate, as described in US 2008/0272378. FIGS. 1A and 1B/B′ illustrate a method of fabricating such a substrate in the prior art.
Referring to FIG. 1A, a starting substrate 100 having thereon an array of crystal piers 110 is provided, and a mask layer 120 is formed on entire sidewalls of the crystal piers 110 and on the starting substrate 100 between the crystal piers 110, exposing the top surfaces 112 of all the crystal piers 110. The crystal piers 110 can be formed by forming an epitaxy layer on the starting substrate 100 and then patterning the same.
Referring to FIG. 1B, an epitaxy process is performed with hydride vapor-phase epitaxy (HVPE) or metallo-organic chemical vapor deposition (MOCVD) to grow an epitaxial layer 140 of the same material from the top surfaces 112 of the crystal piers 110, during which crystal grains firstly grow upward from the top surfaces 112 only due to the masking of the mask layer 120 and then grow upward and laterally to coalesce into a single layer as indicated by the dash lines 142, and the single layer continues to grow upward into a sufficiently thick epitaxial layer. Thereafter, the crystal piers 110 may be broken to provide an epitaxial substrate without a starting substrate.
As shown in FIG. 1B, sometimes polycrystalline nuclei are randomly deposited in some gaps 130 between the crystal piers 110 before the coalescence of the crystal grains and grow into polycrystalline grains 144 that fills up the gaps 130 or even makes the required layer 140 have polycrystals, as described in U.S. Pat. No. 6,582,986.
As shown in FIG. 1B′, some crystal grains may not only grow upward and laterally as indicated by the arrows 146 but also grow downward as indicated by the arrow 148 before the coalescence, due to the characteristics of the HVPE or MOCVD. Thus, some gaps 130 between the epitaxial layer 140 and the substrate 100 get too small or even vanish.
In addition, since the sidewalls of the crystal piers 110 are entirely covered by the mask layer 120 that usually includes a dielectric material being hard and having no cleavage plane, the crystal piers 110 are difficult to break so that an epitaxial substrate without a starting substrate is difficult to obtain.